Recent Research Highlights
Comprehensive study of the glass-like structural ordering in molecular conductors κ-(BEDT-TTF)2X
We have studied the low-frequency dynamics of the charge carriers in different organic chargetransfer salts κ-(BEDT-TTF)2X with polymeric anions X by using resistance noise spectroscopy. Our aim is to investigate the structural, glass-like transition caused by the conformational degrees of freedom of the BEDT-TTF molecules’ terminal ethylene groups. Although of fundamental importance for studies of the electronic ground-state properties, the phenomenology of the glassy dynamics has been minimally investigated and its origin is not understood. Our systematic studies of fluctuation spectroscopy of various different compounds reveal a universal, pronounced maximum in the resistance noise power spectral density related to the glass transition. The energy scale of this process can be identified with the activation energy of the glass-like ethylene endgroup structural dynamics as determined from thermodynamic andNMRmeasurements. For the first time for this class of ‘plastic crystals’,we report a typical glassy property of the relaxation time, namely a Vogel–Fulcher–Tammann law, and are able to determine the degree of fragility of the glassy system. Supporting ab initio calculations provide an explanation for the origin and phenomenology of the glassy dynamics in different systems in terms of a simple two-level model, where the relevant energy scales are determined by the coupling of the ethylene endgroups to the anions.
Origin of the glass-like dynamics in molecular metals κ-(BED-TTF)2X: implications from fluctuation spectroscopy and ab initio calculations, New. J. Phys. 17, 083057 (2015) (open access)
Lorentzian spectra indicating two-level fluctuations in the vicinity of the glass-like structural ordering of the BEDT-TTF molecules' terminal ethylene groups.
Building block of Co nanoisland artificial square spin ice
We present measurements of the thermal dynamics of a Co-based single building block of an artificial square spin ice fabricated by focused electron-beam-induced deposition (FEBID). We employ micro-Hall magnetometry, an ultra-sensitive tool to study the stray field emanating from magnetic nanostructures, as a new technique to access the dynamical properties during the magnetization reversal of the spin-ice nanocluster. The obtained hysteresis loop exhibits distinct steps, displaying a reduction of their "coercive field" with increasing temperature. Therefore, thermally unstable states could be repetitively prepared by relatively simple temperature and field protocols allowing one to investigate the statistics of their switching behavior within experimentally accessible timescales. For a selected switching event, we find a strong reduction of the so-prepared states' "survival time" with increasing temperature and magnetic field. Besides the possibility to control the lifetime of selected switching events at will, we find evidence for a more complex behavior caused by the special spin ice arrangement of the macrospins, i.e., that the magnetic reversal statistically follows distinct “paths” most likely driven by thermal perturbation.
Nanocluster building blocks of artificial square spin ice: Stray-field studies of thermal dynamics, J. Appl. Phys. 117, 17C746 (2015)
Magnetic stray-field studies of a single Cobalt nanoelement as a component of the building blocks of artificial square spinice, J. Magn. Magn. Mat. (published online, 20 August 2015)
We compare the results of an individual building block (nanocluster) of interacting elements in artificial square spin ice with the switching of a single nanoisland. By analyzing the survival function of the repeatedly prepared state in a given temperature range, we find thermally activated switching dynamics. A detailed analysis of the hysteresis loop reveals a metastable microstate preceding the overall magnetization reversal of the single nanoelement, also found in micromagnetic simulations. Such internal degrees of freedom may need to be considered, when analyzing the thermal dynamics of larger spin ice configurations on different lattice types.
Diverging low-frequency fluctuations at the critical endpoint of a Mott transition
We report on the dramatic slowing down of the charge carrier dynamics in a quasi-two-dimensional organic conductor, which can be reversibly tuned through the Mott metal-insulator transition (MIT). At the finite-temperature critical end point, we observe a divergent increase of the resistance fluctuations accompanied by a drastic shift of spectral weight to low frequencies, demonstrating the critical slowing down of the order parameter (doublon density) fluctuations. The slow dynamics is accompanied by non-Gaussian fluctuations, indicative of correlated charge carrier dynamics. A possible explanation is a glassy freezing of the electronic system as a precursor of the Mott MIT.
Critical Slowing Down of the Charge Carrier Dynamics at the Mott Metal-Insulator Transition, Phys. Rev. Lett. 114, 216403 (2015)
So-called second noise spectra provede evidence for non-Gaussian fluctuations at the second-order critical endpoint of the first-order Mott metal-insulator transition. A possible explanation is glassy dynamics of the correlated electrons, where our results indicate a picture, where the system wanders collectively between metastable states related by a kinetic hierarchy.
Spin charge lattice coupling and the CMR effect in EuB6
The coupling of magnetic and electronic degrees of freedom to the crystal lattice in the ferromagnetic semimetal EuB6, which exhibits a complex ferromagnetic order and a colossal magnetoresistance effect, is studied by high-resolution thermal expansion and magnetostriction experiments. EuB6 may be viewed as a model system, where pure magnetism-tuned transport and the response of the crystal lattice can be studied in a comparatively simple environment, i.e., not influenced by strong crystal-electric field effects and Jahn-Teller distortions.We find a very large lattice response, quantified by (i) the magnetic Grüneisen parameter, (ii) the spontaneous strain when entering the ferromagnetic region, and (iii) the magnetostriction in the paramagnetic temperature regime. Our analysis reveals that a significant part of the lattice effects originates in the magnetically driven delocalization of charge carriers, consistent with the scenario of percolating magnetic polarons. A strong effect of the formation and dynamics of local magnetic clusters on the lattice parameters is suggested to be a general feature of colossal magnetoresistance materials.
Lattice Strain Accompanying the Colossal Magnetoresistance Effect in EuB6, Phys. Rev. Lett. 113, 067202 (2014)
Magnetically driven electronic phase separation in the semimetallic ferromagnet EuB6, Phys. Rev. B 86, 184425 (2012)
Combined measurements of fluctuation spectroscopy and weak nonlinear transport of the semimetallic ferromagnet EuB6 reveal unambiguous evidence for magnetically driven electronic phase separation consistent with the picture of percolation of magnetic polarons (MP), which form highly conducting magnetically ordered clusters in a paramagnetic and “poorly conducting” background. These different parts of the conducting network are probed separately by the noise spectroscopy/nonlinear transport and the conventional linear resistivity. We suggest a comprehensive and “universal” scenario for theMPpercolation, which occurs at a critical magnetization either induced by ferromagnetic order at zero field or externally appliedmagnetic fields in the paramagnetic region.
Multiferroicity in an organic charge-transfer salt
Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view. A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order. There, as for conventional ferroelectrics, the electrical dipoles arise from an off-centre displacement of ions. However, recently a different mechanism, namely purely electronic ferroelectricity, where charge order breaks inversion symmetry, has attracted considerable interest. Here we provide evidence for ferroelectricity, accompanied by antiferromagnetic spin order, in a two-dimensional organic chargetransfer salt, thus representing a new class of multiferroics. We propose a charge-order-driven mechanism leading to electronic ferroelectricity in this material. Quite unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly simultaneously. This can be ascribed to the loss of spin frustration induced by the ferroelectric ordering. Hence, here the spin order is driven by the ferroelectricity, in marked contrast to the spin-driven ferroelectricity in helical magnets.
Multiferroicity in an organic charge-transfer salt that is suggestive of electric-dipoledriven magnetism, Nature Materials 11, 755 (2012)
Multiferroicity in the Mott insulating charge transfer salt κ-(BEDT-TTF)2Cu[N(CN)]2Cl, IEEE Trans. Magn. 50, 2700107 (2014)
The proposed multiferroic state of the organic charge-transfer salt k-(ET)2Cu[N(CN)2]Cl has been studied by dc conductivity, magnetic susceptibility and measurements of the dielectric constant in various differently prepared single crystals. In the majority of crystals, we confirm the existence of an order-disorder-type ferroelectric state, which coincides with
antiferromagnetic order. This phenomenology rules out scenarios which consider an inhomogeneous, short-range ordered ferroelectric state. Measurements of the dielectric constant and the magnetic susceptibility on the same crystals reveal that both transitions lie very close to each other or even collapse, indicating that both types of order are intimately coupled to each other. We address issues of the frequency dependence of the dielectric constant epsilon' and the dielectric loss epsilon'' and discuss sample-to-sample variations.